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How can GPS devices be tracked from anywhere in the world?
The Automatic Packet Reporting System (APRS) is a digital communication system for real-time information exchange between amateur radio users in a local area. First developed by Bob Bruninga in the late 1980's, the system has grown to 40,000 users. APRS by Bob APRS handles the relay of many types of information, weather, traffic, alerts, and even text messaging. The most common type of data handled by the network is the GPS position of a mobile station. By using a network of digital repeaters (digipeaters) and internet connected receivers (i-gates) the system is able to relay this information to anywhere in the world without the use of a cellular network. Being a unique communication network, APRS has unique challenges and solutions to problems that will be discussed here. The Short Answer 'Network Overview' The APRS network is formed by a number of nodes covering a local area with each node communicating with its nearest neighbors. Unlike other packet radio transmission protocols, APRS nodes broadcast in an unconnected manner. Rather than establishing a two-way connection between two users, all packets across the APRS network are sent with the ability for all receivers within range to hear. A digital repeater (digitpeater) will forward the packet to all the nodes with its range extending the range of the transmission beyond that which the original transmitter would be able to achieve. The packet will be forwarded several times until a limit set by the user is reached. It is likely that one of the receivers that intercepts the packet will be an APRS Internet Gateway (IGate) which will upload the data to the internet where it will be handled by the APRS Internet protocols and displayed on a website or sent to users conencted to an APRS server. If a given local area has several digital repeaters any message sent will be received at multiple digipeaters and rebroadcast multiple times. These multiple transmissions increase the likelihood that a packet will be received over transmission in a non-broadcast manner. As we will examine, this protocol can also cause problems for the network when used in a dense area. Once uploaded to the internet duplicates of the same packet will be filtered out and the clean result can be displayed on a web page. If the packet is a message intended for another wireless user the APRS-IS network will forward the packet to the nearest IGate from which it will be transmitted over RF to the recipient. 'Packet Anatomy' Packets sent through APRS use the AX.25 protocol designed for amateur radio use. The protocol is able to transfer packets among nodes and detect errors in the communications channel. At each node the protocol is implimented with a Terminal Node Controller, a device containing a microcontroller running software to impliment the protocol and a modem to convert between baseband digital information and modulated audio signals. The TNC is connected to a transciever and is able to provide an interface to a user or be used autonomously to impliment functions like in a digipeater. Many types of data are able to be sent through APRS. Weather, traffic, special alerts, and messages between users are all common transmissions. The network is bet known for its ability to process GPS data from a mobile station and provide the ability to track the station in real time from anywhere in the world. Websites such as findu.com and aprs.fi gather data from internet connected nodes throughout the network and display received GPS packets and the position of the station that sent them. 'Path Settings' Since each digipeater acts autonomously there needs to be a control mechanism built into the packet to control how many times the packet is repeated across the network to prevent endless copies from being made. Each packet contains a path setting in WIDEn-N format where n is the number of times the user wishes the pakcet to be repeated and N is the number of times the packet has been repeated. When the packet is received at a digipeater the path setting is examined. If N=0 no action is taken. If N is not equal to zero, the packet is repeated with N decrimented by 1. We can distinguish between two types of digital repeaters. The first are the high power digipeaters that offer large coverage area and the second are lower power home-based "fill-in" digipeaters that do not have the same range capability as large digis, but can help fill in gaps in network coverage. If we want to first use one of these fill-in repeaters as a step onto the large coverage network we append WIDE1-1 to the end of our path setting. It is also worth noting that many digipeaters will add their callsign to a packet when it is repeated so that we may trace the path that each packet has taken through the network. This helps with determining the georgpahic area that the orginial station is in as well as help us diagose problems in the network as will be seen later. The Long Answer Physical Characteristics The APRS network operates primarily on one frequency, 144.39MHz, making most digipeaters half-duplex. We will assume this is the case for all digipeaters in the following analysis. The data rate on this channel is 1200 baud. Data is encoded using Audio Frequency Shift Keying (AFSK) where data is represneted by shifts in tone of an audio signal. (Listen) Each packet lasts around one second. We usually denote average use in units of packets/30 minutes so we will denote capacity as the same, 1800 packets / 30 minutes. In reality due to physical constraints this maximum might be slightly less. Copies Made By Digipeating As mentioned before, each time a message is received by a repeater it is resent (assuming its path setting counter is not zero). Since the nodes work autonomously, a transmission will be repeated by every digi within its range. Let's assume that the distribution of digipeaters across a local area is symetric. This is a valid approximation since digipeaters tend to be placed not in the immediate area of another, but in such a way as to fill in a gap in the network. We will assume that every digipeater can hear k'' others including the station that sent it the mesage. The calculations will be evaluated at each hop, denoted by ''t where t=1 after one hop, t=2 after two hops, and so on. The first hop will be from the mobile station to all digipeaters within range l1=k The following hops will branch out, increasing the number of copies made with each hop. lt+1=lt+2(k) After N hops have been completed (as determined by the value of n that the user sets in the path setting) the number of copies of the packet that have been introduced into they system will be L = \sum^{N}_{t=1}lt References